Do I have a broken hot water cylinder coil?

[richvdh]

Long story short, I drained the cold water storage tank at the weekend; the hot water cylinder ended up empty too; and by the time I came to refill, I noticed that the pressure in the heating circuit had dropped to near zero.

Advice needed: could anything cause this other than a broken coil?


Some more detail: The boiler's in the loft, along with an expansion vessel for the heating circuit with a pressure gauge. There's a filling loop which is teed off the mains cold supply on its way up to the CWS tank. There's also an automatic air vent, and an overpressure valve which drains outside. The heating circuit is divided into three heating zones plus hot water by electric valves.

We moved into this house about 3 months ago. Soon after, I bled all the radiators. A lot of air came out, and I topped the circuit up to about 1 bar afterwards. Since then I've noticed maybe a couple of times that the pressure was getting a bit low again, but there aren't any visible leaks, so until this weekend I assumed it was just more air working its way out of the system.


Anyway, on Saturday I shut the supply off to the CWS tank to service the float valve. That turned out to be more of a job than expected and it wasn't until Sunday afternoon (by which time we'd used up all the water in the CWS tank) that I got to refill it - by which point, as I said, I noticed that the pressure had dropped to near zero - and I'm pretty sure that it was fine before I started.


I've not noticed any discolouration in the hot water, but what else could cause this other than a broken coil? I'm hoping it's not that, because (a) I could do without the expense, and (b) somebody has built the HWC in behind the bathroom wall so it's going to be a pita to change.

 

[OwainDIYer]

Your hot water cylinder is open vented, but the coil/radiators are pressurised?

Even just 1 bar pressure = head of 10 metres, so unless you have quite a tall house the pressure in the vented cylinder will still always be less than the pressure in the coil, therefore shouldn't be able to stop the boiler depressurising in the event of a coil leak. If there were a coil leak then this would be letting through into the vented system which would eventually then overflow at the tank (and you'd probably have mucky hot water) especially overnight when hot water isn't drawn off and the storage tank is full.

I suppose it's possible there's a leak (or PRV [overpressure valve] is letting through) which is balanced by the filling loop also letting through, and whenyou turned off the cold water you stopped feeding the filling loop. Or some other fault or combination of faults.

It's not impossible there's a coil leak but there are several pointers against it. I'd repressurise the boiler and put a plastic bag over the PRV outlet to catch and show any slow leak there. Likewise refill the vented circuit and put a plastic bag over the tank overflow. Draw off no hot water and see if the tank level rises. If you're really worried about a coil leak inject some fluorescein tracer dye into the radiator circuit through a bleed valve - that will be really visible if it gets into the hot water.

 

[richvdh]

Thanks for the advice Owain.

Your hot water cylinder is open vented, but the coil/radiators are pressurised?

Yes, exactly.

Even just 1 bar pressure = head of 10 metres, so unless you have quite a tall house the pressure in the vented cylinder will still always be less than the pressure in the coil, therefore shouldn't be able to stop the boiler depressurising in the event of a coil leak.

Yup, fair point. It might slow the leak down a bit though?

I suppose it's possible there's a leak (or PRV [overpressure valve] is letting through) which is balanced by the filling loop also letting through, and whenyou turned off the cold water you stopped feeding the filling loop.

Interesting idea. It's possible the PRV is letting through a bit, but it all seems dry. I've disconnected the outlet pipe and put a bag over the end to see if anything comes out. I've also turned off that cold supply again to see if it happens again.

If you're really worried about a coil leak inject some fluorescein tracer dye into the radiator circuit through a bleed valve - that will be really visible if it gets into the hot water.

Good idea.

 

[richvdh]

So I turned off that cold supply again, and let the tank drain down a bit. It was probably off for about 12 hours, and of course absolutely no change to the pressure in the heating circuit.

My bag over the PRV outlet stayed bone dry too.

So the mystery of why it's sometimes dropping pressure remains. Guess I'll just keep an eye on it for now!

 

[muggles]

What boiler do you have?

 

[richvdh]

Hi muggles. It's an Ideal Mexico 2 CF125.